For the purposes of fault isolation in a datapath, it is desirable to detect the loss of transmission units (e.g. ATM cells, IP packets, etc), the addition of transmission units, and/or repeated transmission of transmission units. To remedy fault conditions such as loss, addition and/or repetition, it is necessary to locate the fault such as at a failed field replaceable unit (FRU) so that the failed FRU can be replaced.
In an ATM switch, for example, buffers and switch fabrics are part of the datapath through the switch. Under certain fault conditions, it is possible for a first-in-first-out (FIFO) buffer to fail in a manner that causes the FIFO buffer to repeatedly transmit a particular ATM cell. However, because in most cases the repeated copies of the particular ATM cell are not corrupted in any way, it is difficult to determine that a fault exists. Additionally, it is also possible for a fault condition in the buffers and switch fabrics to cause ATM cells to be lost as they traverse the datapath. However, because a mechanism for detecting cell loss is not provided in the ATM protocol, it is difficult to determine that a fault exists that is causing the ATM cells to be lost.
Generally speaking, detection and fault isolation of transmission unit loss/addition is much more difficult to identify than is detection of transmission unit corruption. This is because a corrupted transmission unit is typically available for inspection and analysis for showing such corruption. Detection of transmission unit loss/addition requires analysis of the transmission unit stream, rather than an individual transmission unit.
A conventional solution for evaluating fault detection in an ATM datapath includes installing a test apparatus in the ATM datapath. The test apparatus includes counters that are inserted at ingress and egress ends of the ATM datapath for counting a number of ATM cells traversing these ends. After a predetermined interval of test ATM cell traffic, the test ATM cell traffic in the datapath is stopped, thus allowing the counters to settle. Once the counters have settled, the values of the counters are compared to determine if cell loss or cell addition has occurred.
One limitation of such a conventional solution is that it is reactive rather than proactive. That is, a fault condition can only be detected when the test apparatus is in place. Another limitation is that subscriber ATM cell traffic in the datapath is stopped during flow of the test ATM cell traffic, thus adversely affecting subscriber services using the datapath. Limitations such as these make this type of conventional solution essentially incompatible with real-time background diagnostic monitoring of a datapath.
Therefore, facilitating loss and fault detection of transmission unit in a manner that overcomes limitations associated with conventional transmission unit loss and fault detection functionality is useful.